Hypothalamus Differences in the Male and Female - Anatomy & Physiology
The hypothalamus is inherently female. Testosterone 'defeminizes' the brain during embryogenesis and eliminates the GnRH surge centre in males. The female foetus has no testes to produce testosterone, thus develops a hypothalamic GnRH surge centre.
Defeminizing the Hypothalamus in the Male
Testosterone crosses the blood-brain barrier and is converted into oestradiol. In the brain, oestradiol 'defeminizes' the hypothalamus and the surge centre function is minimized. Complete 'defeminization' of the brain requires postnatal exposure to androgens. Continued exposure to androgens is required to render the surge centre inoperative. As a result, LH does not surge, it maintains a relatively consistent episodic pattern of secretion every day. Episodes occur every 2-6 hours in the postpubertal male. Steady, pulsatile GnRH and LH release results in a steady, pulsatile release of testosterone.
The Female Hypothalamus
Foetal ovaries produce oestradiol, but this does NOT defeminize the hypothalamus. A protein called alpha-fetoprotein binds oestradiol to prevent it from crossing the blood brain barrier, therefore oestradiol cannot affect the hypothalamus. Alpha-fetoprotein is synthesized firstly by the embryonic yolk sac and then by the foetal liver.
Alpha fetoprotein functions as a foetal blood osmotic regulator and a carrier of fatty acids. As GnRH surges, LH and oestradiol surge about every 20 days in the female. Surge frequency varies between species and is dependent on the length of their oestrous cycles. Between surges, there are low amplitude LH pulses.
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